This invention relates to a method and apparatus for determining the density of polyethylene.
Polyethylene can be considered to be a binary mixture of an amorphous and a crystal phase. The degree of crystallinity affects the elastic properties such as elastic modulus, the impact strength and the melting point.
It is known that the density of polyethylene is directly related to the degree of crystallinity and thus the properties of the material. Density is the major criteria for characterizing and classifying polyethylene.
The most commonly used and accepted method for the determination of density is that of the "Density Gradient Column" defined by ASTM D1505 standard. A small piece of the sample is placed in a column of liquid which exhibits a known density gradient. After it has sunk to its equilibrium, the density is obtained by reading its position in the column. The technique and procedure are quite involved and suffer from numerous drawbacks. First the column must be prepared from a mixture of different solutions of known densities which are carefully added on top of one another. This is time consuming and requires skill to perform. The column which must be isolated from shock and vibrations in a constant temperature environment must then be calibrated using standard glass floats. The column having a limited lifetime (2 or 3 weeks) requires the calibration to be verified periodically. The material (powder, granules or pellets) for which the density is to be measured is press molded into plates roughly 15.times.10.times.0.2 cm according to standardized procedures. The actual measurement is done on small pieces (typically 0.3.times.0.3.times.0.2 cm) which are cut out of the plate. The mere action of cutting causes local impression and the pieces being small, their density will be affected. It is often the practice to allow these strains to relax by thermal conditioning. This again takes time. The pieces having been wetted down are gently introduced in the liquid where they settle in roughly 20 minutes. When a certain number of measurements have been made the column must be cleared of the material which is retrieved with a basket taking care not to upset the gradient.
It is generally known that the velocity of sound is related to the density of materials as well as other properties. For example, U.S. Pat. No. 4,327,587 discloses a method and apparatus for the continuous measurement of changes of rheological properties of monomer during polymerization by monitoring the propagation of ultrasonic oscillations. U.S. Pat. No. 4,297,608 discloses measuring equipment for acoustic determination of the specific gravity of liquids.
The velocity (v) at which sound propagates a material is related to the elastic modulus (M) and the density (.rho.) as follows: v=(M/.rho.).sup.1/2. In many usual cases, materials which show large variations of density (.rho.), exhibit comparatively smaller changes of modulus, such that the velocity (v) decreases with increasing density. As an example, Aluminum, Iron, and Silver have densities .rho..sub.A1 =2.71, .rho..sub.Iron =7.8 and .rho..sub.Silver =10.5 g/cm.sup.3 respectively; with corresponding velocities: v.sub.A1 =6.35, v.sub.Iron =4.5 and v.sub.Silver =3.6 km/sec. However, this behaviour is not a general rule, and the velocity may appear independent of .rho., such as comparing Aluminum and Magnesium having densities .rho..sub.A1 =2.71 and .rho..sub.Mag =1.74 g/cm.sup.3 and velocities v.sub.A1 =6.35 and v.sub.Mag =6.31 km/sec. In other cases, the velocity will increase, or decrease even though the densities do not change much: Silver and Molybdenum where .rho..sub.Silver =10.5 and .rho..sub.Moly =10.3 g/cm.sup. 3 while v.sub.Silver =3.6 and v.sub.Moly =6.29 km/sec. Finally the velocity may increase with increasing density such as in going from the Brass to Molybdenum for which .rho..sub.Brass =8.56 and .rho..sub.Moly =10.3 g/cm.sup.3 and v.sub.Brass =4.28 and v.sub.Moly =6.29 km/sec. These examples and many others show that the behaviour for the velocity of sound cannot a priori be predicted from the value of density.
For most solids, the elastic modulus, and thus the velocity does not vary with frequency. Materials for which the elastic modulus is independent of frequency are qualified as being purely elastic materials.
However, there exist other materials, such as polymers, of which polyethylene is an example, which are partly elastic and partly viscous. For such materials which are referred to as visoelastic materials, the velocity is not independent of frequency; at low frequency, the material appears more viscous than at high frequency where it appears elastic. At low frequency, the velocity increases with frequency.